I'll have to look up the source after Shabbos, but my understanding is that the offspring of two people who could never have a legitimate relationship will always be a mamzer. Therefore, the offspring of incest and the offspring of a Kohen and a divorcee are mamzerim.

In your case, the offspring is of a couple who could have a legitimate relationship; however, the conception occurred at an inapropriate time.

What about the offspring of a man and his wife's sister? They can have a permissible relationship after his wife dies. Note also that the product of a kohen and a divorcee is not a mamzer.
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msh210♦Aug 12 '12 at 4:22

Also a man and his brother's wife who might become permitted to him eventually through Yibbum.
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Double AA♦Aug 16 '12 at 15:47

I think both comments miss the obvious example: a married woman. Once she stops being married, she could be with this other fellow.
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Ze'ev FelsenSep 10 '12 at 18:31

1

It's definitely not true that the offspring of a Kohen and divorcee are mamzerim. There's no chaiv kares in the relationship. The marriage is a real marriage, but they are required to get divorced. This is a mishnah in Kesuvos.
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Craig FeinsteinSep 27 '12 at 13:49

@Ze'evFelsen Actually, if someone had a relationship with a married woman they are forever prohibited to each other, even if she divorced or her husband died.
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ArielDec 30 '12 at 9:40